Session Initiation Protocol (SIP) is an application-layer protocol for creating, modifying, and terminating media sessions, such as voice and/or video calls, between participant systems. The SIP protocol is detailed in Rosenberg et. al., “SIP: Session Initiation Protocol,” RFC 3261, June 2002, a copy of which is filed concurrently with this application, and is incorporated by reference in its entirety. Various hardware and software products are available which allow mobile stations, such as smartphones, to use SIP to enable media sessions. For example, a number of devices running the Android operating system, and various applications for smartphone devices, provide such features.